This invention relates generally to improvements in prosthetic devices used for reconstruction of the knee joint in humans. More particularly, this invention relates to an improved tibial component for use in a knee prosthesis, wherein the tibial component provides improved load bearing capability during normal postoperative patient function.
Artificial or prosthetic joint mechanisms for implantation into animals, particularly humans, have been the subject of intensive research and development efforts for many years. Such prosthetic joint mechanisms have typically comprised one or more implant components formed from a relatively biostable material having selected structural properties and a unique shape to replace all or part of a selected anatomical joint, for example, a hip or knee joint. The implant components are installed by surgically accessing the joint and by resection of one or more bone surfaces to accommodate direct attachment thereto of the implant components. In the past, attachment of implant components to patient bone has been commonly achieved by use of bone cements, such as a methyl methacrylate based cement or the like used as a grouting material to fill up the space between the receptive bone surface and the prosthetic component. More recently, however, a variety of structural and biological incompatibility problems encountered with the use of bone cements have led to the development of so-called bone ingrowth materials. In such bone ingrowth materials, a surface coating of controlled porosity is provided on a prosthesis component in a position for intimately contacting patient bone to achieve a significant degree of postoperative bone and/or tissue ingrowth, and thereby obtain a mechanical interlock with patient bone without utilizing bone cement.
The human knee joint has presented particularly difficult problems in the development of a satisfactory prosthetic joint. More specifically, the human knee joint is recognized as an extremely complex mechanical structure which is subjected to high mechanical loads of widely varying magnitude and direction during normal function. Unfortunately, the knee joint is also subject to a relatively high frequency of disabling injury occurrence. As a result, a wide variety of knee prostheses have been proposed in the prior art, typically to include matingly configured femoral and tibial components adapted respectively for implantation onto the lower end of a resected femur and the upper end of a resected tibia, with appropriate plastic meniscal bearing components interposed therebetween. In the majority of these prior art knee prostheses, the general configuration of the femoral and tibial components has resembled the general physiology of the natural knee joint, namely, to include medial and lateral condyles on the femoral component which are supported by the meniscal bearing components on the tibial component.
However, notwithstanding the many knee prosthesis designs which are available in the art, prior knee prostheses have exhibited an unacceptably limited mechanical load bearing capacity and/or have been subjected to an unacceptably high risk of premature failure. Such failure of the prosthesis most commonly occurs by loosening or detachment of the load bearing tibial component relative to the patient's tibia. When this occurs, surgical revision of the reconstructed knee joint is necessary if the patient is to remain of regain any significant level of ambulation. Unfortunately, surgical revision entails undesirable patient trauma, risk of infection, and general disruption and deterioration of the vascular system in the region of the reconstructed knee. Moreover, in many patients, loss or deterioration of bone structure at the knee joint can make revision surgery extremely difficult and frequently impossible. As a result, knee prostheses have been utilized to date on an extremely limited basis.
One of the major problems encountered with knee prostheses is the inability to insure stable and secure fixation of the tibial component onto the upper end of the patient's tibia. More particularly, secure fixation of the tibial component in a permanent manner is crucial to obtaining satisfactory prosthesis performance, since it is the tibial component which must withstand the high and variant mechanical loads typically of a compressive nature, during normal patient movements. In this regard, during implantation surgery, the upper end of the patient's tibia is resected to expose the asymmetric cross section of the tibia defined by a hard outer shell of cortical bone surrounding a softer interior filled with porous cancellous bone. Ideally, a tibial component is selected to have size and shape to rest securely upon the cortical shell without significant overlap outside the cortical margin in any direction. However, it is frequently difficult for surgeon to insure precision placement of the tibial component in a position fully supported by the hard cortical bone. If any portion of the implant periphery is supported by the softer cancellous bone, the implant can experience tipping or subsidence when subjected postoperatively to normal patient loading, with the result that the implant will work loose over a period of time. Moreover, inherent uneven mechanical loading of the tibial component, particularly in the medial-lateral plane, can also cause the implant to tip and work loose over a period of time.
There exists, therefore, a significant need for an improved tibial component for a knee prosthesis, wherein the tibial component is designed to withstand normal bearing loads during normal patient function without working loose relative to patient bone, and particularly wherein the tibial component is designed to accommodate the inherent uneven loading without risking the necessary secure and stable attachment to patient bone. The present invention fulfills these needs and provides further related advantages.